Networks, particularly large networks, may make use of points of presence (PoP) in many locations around the world. These PoPs may be, for example, proprietary networks belonging to a company. Among other things, these PoPs may carry traffic between servers and clients (e.g., end-users).
Internet Service Providers (ISPs) typically announce routes to reach their customers, and possibly of customers of downstream ISPs accessible through the ISP. Some ISPs, called transit or peer ISPs, announce routes to reach, not just their own customers, but to reach other customers as well (e.g., some transit or peer ISPS announce routes to reach anyone on the Internet).
Conventionally, the primary mechanism used to route traffic is the Border Gateway Protocol (BGP), which may run on a router. BGP accepts as input the routes that ISPs announce over various peering links, and chooses a route to use based on standard BGP metrics.